1. Field of the Invention
This invention relates to the fabrication of devices and, in particular, to the fabrication of devices by thermal processes.
2. Art Background
Various methods have been devised for measuring the temperature of an article. Approaches generally rely on either optical or electrical measurements with or without a thermal probe. Generally electrical measurement techniques use a probe such as a thermocouple where the electrical characteristics of a bimetal contact in proximity to the article is indicative of article temperature.
It is possible to perform optical measurements with a probe or directly on the article without a probe. Exemplary of optical probes is a silica optical fiber including a terminal region of silicon. (See U.S. Pat. No. 4,437,761 dated Mar. 20, 1984). The refractive index of the silicon region varies strongly with temperature and thus light traversing the optical fiber and incident on the silicon region is reflected both at the interface of the silicon region with the silica fiber and at the interface of the silicon region with the ambient. The resulting interference pattern between the two portions of reflected light, due to the strong temperature dependence of the refractive index in silicon, and to a significantly lesser extent to thermal expansion, allows a measure of temperature. However, it must be assumed that the electrical or optical probe and the article are at the same temperature. Even when the probe contacts the article to be measured, such assumptions are often at best approximate.
As discussed, direct measurement techniques not requiring the assumption of temperature equivalence between probe and article are also available. Exemplary of these techniques is a process described by D. Hacman in Optik, 28, 115 (1968). In this technique, the temperature of a quartz substrate is monitored by directing visible light onto the surface of the substrate. Likewise, R. A. Bond, et al. in Journal of Vacuum Science and Technology, 18, (2), 335 (1981) have used this technique to measure the temperature of a quartz substrate in a plasma reactor. As in the previously described optical fiber technique, interference occurs due to reflection at both the incident surface of the glass and at the remote substrate surface. Since the coefficient of linear expansion of the substrate is temperature dependent, a monitoring of the interference pattern gives a measure of the change in substrate thickness and thus the associated temperature change. Similarly, pyrometric techniques are also available which do not depend upon an assumed equivalence between article and probe temperatures. In these measurements, black body radiation characteristic of temperature is emitted by the article and is detected.
The quality of devices such as optical devices, electronic devices and optoelectronic devices depends, to a large extent, on the control of processes employed in their fabrication. A significant processing condition in most such procedures is the temperature. For example, in deposition techniques where a heated substrate is subjected to gases that undergo thermally induced chemical reactions to produce deposition on the substrate, the substrate temperature significantly affects the composition of the resulting deposit. Exemplary of such deposition techniques are molecular beam epitaxy (MBE), chemical vapor deposition (CVD) and metal organic chemical vapor deposition (MOCVD). (A description of these processes can be found in Chang and Ploog, Molecular Beam Epitaxy and Heterostructures, Martinus Nijhoff Publishers, Dordrecht, 1985, Journal of Crystal Growth, 55 (1981) and Chemical Vapor Deposition for Microelectronics by A. Sherman, Noyes Data Corporation, Parkridge, N.J., 1987. In general, these techniques all depend on the interaction of gas phase entities with a heated substrate to produce deposition.) Similarly, etching processes such as plasma etching and reactive ion etching (RIE) also depend on substrate temperature. For example, if the temperature across a wafer varies significantly, the etch rate across the substrate also differs. Clearly, a spatial variation in etch rate across a substrate will produce undesirable nonuniformities during fabrication.
Presently, for processes such as MBE, MOCVD, and CVD the more accurate the substrate temperature measurement the better the control of the process. Techniques such as plasma etching and reactive ion etching (RIE) can be advantageously controlled presently without temperature monitoring. However, as device structures become smaller, the effects of temperature are expected to produce unacceptable nonuniformities even in these etching techniques. Therefore, precise temperature monitoring is quite significant.
As a result, techniques relying on the weak assumption of temperature equilibration between the substrate and temperature probe are not desirable. Techniques such as optical pyrometry also involve significant inaccuracies. Optical pyrometry depends on the measure of absolute intensity of light emitted by a substrate. This absolute intensity is strongly affected by properties such as transmittance of optical windows in the fabrication chamber and emissivity of the substrate itself. Since it is generally expected that these parameters will significantly change during processing, i.e. unavoidable contamination will be expected to change window transmission and substrate surface change will be expected to effect substrate emissivity, the measure of absolute intensity is inaccurate at best.
Techniques involving the monitoring of changes in the linear coefficient of expansion with temperature, although initially suggested for monitoring of device processing, have not been pursued. This lack of activity has possibly occurred due to inaccuracies inherent in the relatively small change of expansion coefficient with temperature. Irrespective of the reasons, a satisfactory technique for temperature monitoring associated with device processing is not presently available.